PrintWriter in java giving unexpected behaviour

Question

import java.io.*;
class demo
{
public static void main(String args[])
{
    PrintWriter pw=new PrintWriter(System.out);
    pw.println("java");
    //pw.print("java");
}
}

// the output is java using pw.println but output is null using pw.print ie nothing gets printed on console while using print .

Answer 1

It's almost certainly just buffering - and as you're not flushing it, you never get the output. From the docs :

Unlike the PrintStream class, if automatic flushing is enabled it will be done only when one of the println, printf, or format methods is invoked, rather than whenever a newline character happens to be output. These methods use the platform's own notion of line separator rather than the newline character.

Try:

pw.flush();

at the end of the code.

Answer 2

Try this instead :

PrintWriter pw=new PrintWriter(System.out);
pw.print("java");
pw.flush();

The PrintWriter is going to be doing internal buffering, and the println method is automatically flushing it.

Answer 3

对于自动刷新，您可以使用此构造函数

PrintWriter(OutputStream out, boolean autoFlush);

Answer 4

A call to println() implicitly flushes the output buffer whereas a call to print() does not. Try using print() and then call pw.flush() .

Note also that there are constructors of PrintWriter which include an option to automatically flush after any write call.